This invention relates generally to telephone networks and more specifically to a method of locating a fault in the network.
Telephone networks contain at least one pair of wires which connects each subscriber to the network, called a "subscriber line". Traditionally, all of the pairs of wires in one geographic region run to a central office. The central office, sometimes called a class 5 office, contains electronic circuitry which is called a "switch."
The switch is also connected to trunk telephone lines, which run to other telephone company switches in the network. To complete a call, the switch in the central office connected to the subscriber placing the call connects the pair of wires running to the subscriber to a trunk line. Through the telephone network, that trunk line is connected to a switch in the central office near the party receiving the call. The switch in the receiving office connects that trunk line to the pair of wires which runs to the subscriber who is to receive the call.
In this way, the trunk lines of the network can be switched to carry calls from different subscribers at different times. Great efficiencies can be achieved by routing telephone calls over trunk lines in the network. Also, the network usually contains more trunk lines than are needed at any given time. If one trunk line is faulty, calls can be rerouted through other trunk lines in the network until the faulty line is fixed.
Nonetheless, at some point, there must still be a pair of telephone wires connecting each subscriber to a switch. As these pairs of wires are dedicated for use by that subscriber, if there is a fault in that pair, the subscriber loses telephone service or receives degraded service. Thus, it is very important for telephone companies to keep these lines in good repair. If faults occur in these subscriber lines, the telephone company needs to repair them as quickly as possible. Quick repair is, unfortunately, very difficult because of the large amount, in linear feet, of subscriber lines. A local telephone company might have millions of telephone subscribers, each on average several miles from the central office. As a result, there are millions of linear feet of subscriber lines. When a fault occurs, before it can be repaired, it must first be located within this millions of feet of wires.
To deal with this problem, most telephone companies use automated test equipment to help identify faults. One example of such test equipment is the 4TEL.RTM. product sold by Teradyne, Inc. of Deerfield, Ill., USA. The 4TEL.RTM. product includes a test and measurement unit (TMU) installed in the central office and is connected to the switch. The TMU can access individual subscriber lines through the switch. The TMU contains several measurement devices which it uses to measure various parameters of the subscriber line.
The measured line parameters are sent to a test system controller (TSC), which is usually located at the telephone company service center. The TSC is, in essence, a computer which has been programmed to analyze the parameters measured for each line and to control which lines are measured at which time. The TSC compares the measured parameters for each line to values which would be expected for a good line. If the measured values are outside of the acceptable range, a fault is detected.
When a fault is detected, the telephone company dispatches a repair person to find and fix the fault. Historically, telephone companies have employed three types of repair people: those who repair faults in the central office; those who repair faults in cables routing the pairs of subscriber lines around the telephone company's service area; and those repairing faults at the "station." The station refers to the subscriber line as it leaves the cable and runs into the customer's premises. The different types of repair people carry different types of equipment and have different types of training geared towards the types of conditions they are likely to encounter. For example, a repair person dispatched to repair a cable might carry a time domain reflectometry (TDR) unit. This unit is connected to a telephone line and transmits an electrical pulse down the line. When the pulse reaches an open circuit or a short circuit fault, it is reflected back towards the TDR unit. By measuring the time it takes for the pulse to travel back and forth to the fault, the distance between the fault and repair person can be computed. In this way, faults can be located to within a section of cable.
To facilitate dispatch of the appropriate personnel, current automatic telephone line test equipment reports whether the fault is most likely in the central office, the cable or the station. This segmentation of faults is performed by observing which of the measured parameters of the line deviated from the expected values. Historically, faults in different locations often produce parameters in different ranges.
However, merely segmenting faults into one of three locations is not sufficient. Specifically, the station includes wires which are owned by the telephone company and wires which are owned by the subscriber. The telephone company owns and is responsible for repairing the "drop." The drop is the connection from the cable to the subscriber's premises. The subscriber owns and is responsible for repairing the wires inside the subscriber's premises. The drop is usually separated from the premises wiring by a network interface unit, which is mounted on the side of the customer premises.
When the telephone company dispatches a repair person to repair a fault at the station, they can not be sure if the fault will be the telephone company's responsibility to repair or that of the subscriber. When the fault is eventually determined to be in the premises wiring, the fault might not be repaired because the repair person has no access to the premises. Additionally, there are issues of allocating the costs of the service call. When the fault is within the premises, the telephone company is entitled to charge the subscriber for the service, if the subscriber has agreed in advance to pay for it. To get advance agreement from the subscriber, though, requires that the telephone company first make a service call to determine whether the fault is in the premises wiring, which is costly to the telephone company. Alternatively, the telephone company must advise the subscriber that there might be a charge for service based on what the repair person finds. Many subscribers object to being informed in this fashion that they might be held responsible for charges.
As a result, there has been a great interest in developing a way to identify whether a station fault is in the drop or within the customer premises. One widely considered solution is the use of Remote Isolation Devices (RID). The RID is a remotely controlled switch which is installed at the interface between the premises wiring and the drop. When it receives a command from the automatic line test system, it disconnects the premises wiring from the drop. The automatic line tester retests the line with the premises wiring disconnected. If the fault persists, it can be localized to the drop, or at least excluded from being within the premises wiring.
Because each subscriber line must include its own RID, each telephone company would have to buy between tens of thousands and millions of RIDs. Significant efforts have been expended designing low cost RIDs. However, even with RIDs that cost as little as a few dollars, the cost of outfitting all subscriber lines is still very large. A service call must be made to each subscriber premises to install the units. Moreover, existing line test equipment would have to be modified to operate in conjunction with the RIDs. Only a few telephone companies have committed to this expenditure. As a result, there still remains a significant need to be able to segment faults on subscriber lines, at least to the level that the telephone company can know with high confidence whether the fault is in the premises wiring before a repair person is dispatched.